Stress sensor for in-situ measurement of package-induced stress in semiconductor devices

ABSTRACT

A stress sensor is disclosed herein. The stress sensor includes a plurality of carbon nanotubes in a substrate, and first and second contacts electrically connectable with the plurality of carbon nanotubes. Methods of making and using the stress sensor are also disclosed.

This is a Divisional Application of Ser. No. 11/523,835 filed Sep. 19,2006 now U.S. Pat. No. 8,174,084, which is presently pending.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor structures andmanufacturing. In particular, the present invention relates to a stresssensor for semiconductor structures.

BACKGROUND OF THE INVENTION

Advances in semiconductor manufacturing technology have led to theintegration of billions of circuit elements, such as transistors, on asingle integrated circuit (IC). In order to integrate increasing numbersof circuit elements onto an integrated circuit it has been necessary toreduce the dimensions of the electronic devices (e.g., ametal-oxide-semiconductor (MOS) transistor).

A typical packaged integrated circuit unit includes a die in or on whichthe integrated circuit is formed and a package substrate on which thedie is mounted. An interconnect structure connects the terminals of thedie from the integrated circuit in the die to the terminals of thepackage, which can be further connected to other components through acircuit board. The package may be directly mounted on the circuit board,or through a socket or an interposer.

Semiconductor manufacturers desire an understanding of thestress-induced performance and reliability effects for these scaledelectronic devices. Destructive techniques are typically used to measurestress levels in the die. Strain gauge rosettes have been considered forplacement on the die, but are not sufficiently accurate or robust atthese smaller scales. Thus, there are no current non-destructive methodsto accurately determine stress levels in semiconductor devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a perspective view of a substrate having a stress sensoraccording to an embodiment of the invention;

FIG. 2 is a side cross-sectional view of a substrate having a stresssensor according to an embodiment of the invention;

FIG. 3 is a side cross-sectional view of a substrate according to anembodiment of the invention;

FIG. 4 is a side cross-sectional view of resist formation on thesubstrate according to an embodiment of the invention;

FIG. 5 is a side cross-sectional view of mask formation and resistdevelopment exposure according to an embodiment of the invention;

FIG. 6 is a side cross-sectional view of a dry etch of the substrateaccording to an embodiment of the invention;

FIG. 7 is a side cross-sectional view of a first resist clean accordingto an embodiment of the invention;

FIG. 8 is a side cross-sectional view of a second resist clean accordingto an embodiment of the invention;

FIG. 9 is a side cross-sectional view of deposition of metal contacts inthe substrate according to an embodiment of the invention;

FIG. 10 is a top view of the metal contacts deposited in the substrateaccording to an embodiment of the invention;

FIG. 11 is a top view of the carbon nanotubes deposited in a trenchbetween the contacts in the substrate according to an embodiment of theinvention;

FIG. 11A is a schematic view of non-aligned carbon nanotubes in asuspension according to an embodiment of the invention;

FIG. 11B is a schematic view of aligned carbon nanotubes in a suspensionaccording to an embodiment of the invention;

FIG. 11C is a schematic view of aligned carbon nanotubes in a fiberaccording to an embodiment of the invention;

FIG. 12 is a side cross-sectional view of deposition of a passivationlayer over the trench according to an embodiment of the invention;

FIG. 13 is a side cross-sectional view of deposition of an interlayerdielectric (ILD) over the passivation layer according to an embodimentof the invention;

FIG. 14 is a side cross-sectional view of deposition of a passivationlayer over the ILD according to an embodiment of the invention;

FIG. 15 is a side cross-sectional view of formation of contactextensions according to an embodiment of the invention;

FIG. 16 is a side cross-sectional view of deposition of a top layeraccording to an embodiment of the invention;

FIG. 17 is a side cross-sectional view of deposition of a contact padover the contact according to an embodiment of the invention; and

FIG. 18 is a schematic view of a four terminal structure for measuringresistance of the nanotubes.

DETAILED DESCRIPTION

A stress sensor, methods of making the stress sensor and methods ofusing the stress sensor are disclosed herein. The stress sensor uses, inone embodiment, carbon nanotubes that are embedded in a surface of asubstrate of the die. When stress is induced in the die, the nanotubesrespond to the stress. The response of the nanotubes can be measured andcorrelated to a stress value.

As shown in FIG. 1 of the accompanying drawings, a stress sensor isincluded in a die. The die may include any well-known substrate ormaterial on which integrated circuits are typically formed.

The stress sensor includes a substrate 10, a trench 12, nanotubes 14 andfirst and second contacts 16, 18. The trench 12 is formed between thefirst and second contacts 16, 18 and in the substrate 10. The nanotubes14 are deposited in the trench 12 between the first and second contacts16, 18. In one embodiment, the nanotubes are aligned in the samedirection as the length of the trench 12. In one embodiment, thenanotubes 14 form a conductive path.

Any well-known substrate, such as, but not limited to, singlecrystalline bulk silicon may be used. In one embodiment, the substrate10 is a silicon wafer. The substrate 10 may be a silicon-on-insulatorstructure. The substrate 10 may be formed from other materials, such as,but not limited to, germanium, indium antimonide, lead telluride, indiumarsenide, indium phosphide, gallium arsenide, gallium antimonide and thelike.

The trench 12 is for holding the nanotubes 14. In one embodiment,several trenches are formed in the substrate 10. In one embodiment,several hundred trenches are formed in the substrate 10. In oneembodiment, a cross-section of the nanotubes in the trench includes afew hundred nanotubes.

The trench 12 may be any depth, length and/or width. In one embodiment,the depth of the trench 12 is similar to the depth of a transistor in asubstrate.

The nanotubes 14 are provided in the substrate 10 to respond to stressinduced in the substrate 10. The nanotubes 14 show a change in voltageacross their length in response to stress because the stress causeschanges in the bond length and angle, giving rise to changes in theirband structure and band gap.

The nanotubes 14 may be metallic, dielectric or semiconductor nanotubes.The nanotubes 14 may be carbon nanotubes. In one embodiment, the carbonnanotubes can be either single walled carbon nanotubes (SWCN) ormulti-walled carbon nanotubes.

The nanotubes 14 may be deposited as a film or form a film which iswithin the trench.

First and second contacts 16, 18 measure the current-voltage response inthe nanotubes 14. The contacts 16, 18 are conductors.

The first and second contacts 16, 18 may be any appropriate metal thatcan make ohmic contact with the nanotubes 14, such as, for example,carbide forming metals. Exemplary carbide forming metals include Ni andTi. In another embodiment, the contacts 16, 18 are aluminum, silver,copper or gold.

FIG. 2 shows the stress sensor embedded in a die.

The die includes the substrate 10, trench 12, nanotubes 14, first andsecond contacts 16, 18, a passivation layer 20, an interlayer dielectric(ILD) 22, a top layer 24, first and second contact extensions 26, 28 andfirst and second contact pads 30, 32.

Passivation layer 20, ILD 22, top layer 24, and contact pads 30, 32 maybe any well-known material and deposited using well-known techniques.Contact extensions 26, 28 are typically the same material as contacts16, 18 and are formed using well-known techniques. In one embodiment,the passivation layer 20 is SiN, ILD 22 is SiO2, and top layer 24 isSiN. In one embodiment, the contact pads 30, 32 are WB pads.

FIGS. 3-17 illustrate a method of making the stress sensor in accordancewith one embodiment of the invention.

As shown in FIG. 3, the process begins by providing a substrate 10.

As shown in FIG. 4, the process continues by forming a resist layer 34over the substrate 10. The resist is formed using well-known techniques.

As shown in FIG. 5, the process continues by forming a mask 36 over theresist layer 34. The illustrated mask has first, second and thirdopenings 38, 39 and 40 therein. The first opening 38 corresponds to thetrench 12 and the second and third openings 39, 40 correspond to thefirst and second contacts 16, 18. It will be appreciated that the maskmay include multiple openings corresponding to multiple trenches andcontacts associated with each trench.

As shown in FIG. 6, the process continues by etching the substrate 10 toform first, second and third openings 42, 43 and 44 in the substrate 10.The first opening 42 corresponds to the trench 12 and the second andthird openings 43, 44 correspond to the first and second contacts 16,18. As described above, the substrate 10 may include multiple openingscorresponding to multiple trenches and contacts associated with eachtrench.

The etching process may be a conventional dry etch process or ananisotropic wet etch, or other known etching techniques.

As shown in FIG. 7, the process continues by removing the mask and theresist, as shown in FIG. 8. When the mask and resist have been removed,the substrate includes openings corresponding to the openings in themask for the trench(es) and associated contacts, as described above. Themask and resist may be removed using well-known techniques.

As shown in FIG. 9, the process continues by forming the contacts 16, 18in the openings 43, 44 in the substrate 10. FIG. 10 shows the contacts16, 18 from a different perspective (top view). In one embodiment, thecontacts are conductors formed by, for example, chemical vapordeposition (CVD), plasma vapor deposition (PVD) or plating.

As shown in FIG. 11, the process continues by depositing nanotubes 14between the first and second contacts 16, 18.

Although the process has been described as deposition of the contacts16, 18 followed by deposition of the nanotubes 14 into the trench 12, itwill be appreciated that the process may include deposition of thenanotubes 14 into the trench 12 followed by deposition of the contacts16, 18.

The process for depositing the nanotubes will now be described infurther detail and with reference to FIGS. 11A-11C.

The trench 12 may be formed using a conventional dry etch process or ananisotropic wet etch, as described above.

The nanotubes may be integrated onto the silicon surface in a number ofways. As discussed above, multiple nanotubes should contact each other,to form an electrically conducting path, and the nanotubes should bealigned in the direction of the trench.

In one embodiment, nanotubes are dispensed directly into the trench 12from their suspension. The suspension of nanotubes is shown in FIG. 11A.In one embodiment, a pipette is used to dispense the nanotubes 14 intothe trench 12. A uniform nanotube suspension is typically made in asolvent, such as, acetone or toluene, by ultrasonication. The nanotubesare then dispensed into the trench using, for example, a pipette. Asolvent evaporation follows the deposition. Multiple dispenses may berequired to ensure the trench is filled with aligned nanotubes. Anynon-aligned nanotubes and extra nanotubes may be removed. In oneembodiment, the non-aligned and/or extra nanotubes are removed byscraping them off with a Cu wire swept over the trench 12.

In another embodiment, the nanotubes are aligned in a suspension beforedeposition of the nanotubes 14 into the trench 12. The suspension may bethe same as described above with reference to FIG. 11A. An electricfield may be applied to a container having the nanotubes and thesuspension to align the nanotubes in the suspension, as shown in FIG.11B. In one embodiment, the electric field is created by placing twoelectrode plates on either side of the container that contains thenanotube suspension.

The die 10 can then be dipped into the nanotubes suspension. In oneembodiment, the trench is aligned in the direction of the electricfield. The die is then removed from the suspension and dried toevaporate the solvent. The process may be repeated multiple times untilthe trench is completely filed with aligned nanotubes.

In a further embodiment, an electro-spun fiber containing alignednanotubes can be formed, as shown in FIG. 11C. The fibers may becomposite polymer-nanotube fibers. In one embodiment, the fibers areabout 100 nm in diameter and a few hundred microns in length. Thenanotubes are aligned in the direction of the fiber axis.Electro-spinning begins with a solution of nanotubes and polymer in asolvent. The solvent is sprayed onto a plate under the application of anelectric field. The field aligns the nanotubes, and the spray atomizesthe solution. The solvent is evaporated and the resulting material is apolymer fiber containing nanotubes aligned in the direction of thefiber. The fibers are packed in the trench, such that the fiber axis isaligned with the trench axis. The substrate may optionally be heated toremove the polymer and expose the nanotubes. The polymer can also beremoved by applying a solvent to dissolve the polymer to expose thenanotubes.

A passivation process may follow deposition of the aligned nanotubes inthe trench. Any passivation technique may follow, including, forexample: 1) a single thick SiN layer; 2) a thin SiN with SiO2 as an ILDand a top SiN layer; 3) a SiN with SiO2, a top SiN and polymide (orother polymer); and the like. An exemplary passivation process will nowbe described with reference to FIGS. 12-17.

After the nanotubes are aligned in the trench 12, the process continuesby forming a passivation layer 20 over the nanotubes 14 and die 10, asshown in FIG. 12.

As shown in FIG. 13, the process continues by forming an ILD 22 over thepassivation layer 20.

As shown in FIG. 14, the process continues by depositing a passivationlayer 21 over the ILD 22.

As shown in FIG. 15, the process continues by forming the contactextensions 26 in the passivation layers 20, 21 and ILD 22.

As shown in FIG. 16, a top layer 24 is formed over the contactextensions 26 and ILD 22.

As shown in FIG. 17, the process continues by forming contact pads 30 onthe contact extensions 26.

In use, a load is applied to the substrate 10. In one embodiment, theload is applied by bending the substrate 10. Application of the loadinduces stress in the die, which is measurable by the stress sensor,described above with reference to FIGS. 1 and 2. In particular, thenanotubes in the trench respond to the stress on the die, and theresponse of the nanotubes can be converted into a stress measurement.

The nanotubes 14 have an average resistance which is a function of theirgeometry. As the nanotubes experience changes in stress within the diedue to temperature and/or pressure changes, the resistance of thenanotubes in the trench changes according to the piezo-resistiveresponse of the nanotubes. By monitoring this resistance, the stress canbe determined, using, for example, the following equation:

$\begin{matrix}{\frac{\Delta\; R}{R} = {K\; ɛ}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where ΔR is the change in resistance, R is resistance, K is a gaugefactor for the film, and ∈ is stress/strain.

The stress sensor is calibrated based on known strain/deflections for adie to determine the gauge factor K.

In one embodiment, the nanotubes 14 are single walled carbon nanotubes(SWCN). Using a common four point bend measurement and equations forstrength of material, the values of resistivity for a SWCN transferredinto stress on bulk silicon are approximately about 250 kPa for a 20-25nm deflection. The equation for calculating the stress can be determinedby the following equation:

$\begin{matrix}{{\sigma = {{ɛ \cdot 131}({GPa})}}{ɛ = \frac{3h\;\delta}{a\left( {{3L} - {4a}} \right)}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where L is the length between the two contacts, a is the distance fromthe load point application to a fixed boundary, h is the thickness ofthe trench; δ is the deflection of the nanotubes, ∈ is strain, and σ isstress.

A typical four terminal structure may be used to measure the electricalresponse of the nanotubes 14, as shown in FIG. 18. Voltage is applied toform a current, and the change in current is measured (i.e., electricalresistivity) to determine the strain in the nanotubes 14 when a load isapplied to the substrate. It will be appreciated that other well-knowntechniques for measuring strain based on electrical response may also beused with the stress sensor described herein.

In one embodiment, the strain is determined from the change inresistance and the stress is calculated, as described above. In oneembodiment, an empirical table is used to determine the stress in thedie from the load applied and resistance response.

Embodiments of the present invention are advantageous because the stresssensor can be incorporated into the die without damaging the originalstress distribution in the die. In addition, because of theiranisotropic structure and piezoelectric properties, nanotubes show anelectrical signal response to stresses in specific directions. Carbonnanotubes sensors are also robust at small sizes.

The methods which are described and illustrated herein are not limitedto the exact sequence of acts described, nor are they necessarilylimited to the practice of all of the acts set forth. Other sequences ofevents or acts, or less than all of the events, or simultaneousoccurrence of the events, may be utilized in practicing the embodimentsof the present invention.

The foregoing description with attached drawings is only illustrative ofpossible embodiments of the described method and should only beconstrued as such. Other persons of ordinary skill in the art willrealize that many other specific embodiments are possible that fallwithin the scope and spirit of the present idea. The scope of theinvention is indicated by the following claims rather than by theforegoing description. Any and all modifications which come within themeaning and range of equivalency of the following claims are to beconsidered within their scope.

The invention claimed is:
 1. A method of making a stress sensor comprising: forming a trench having a length in a substrate of a die; depositing a plurality of carbon nanotubes in the trench, the carbon nanotubes aligned in the direction of the length of the trench wherein depositing the plurality of carbon nanotubes in the trench comprises: forming a nanotube suspension in a solvent; dispensing the nanotube suspension into the trench using a pipette; evaporating the solvent; removing non-aligned nanotubes; and forming first and second contacts on respective first and second ends of the trench.
 2. The method of claim 1, wherein forming the trench in the substrate comprises: forming a photoresist mask on the die, the photoresist mask having an opening; and etching the die in combination with the photoresist mask to form the trench.
 3. The method of claim 1, wherein depositing a plurality of carbon nanotubes in the trench comprises: forming a nanotube suspension in a solvent; applying an electric field to the solvent to align the nanotubes; and dipping the substrate into the solvent, the trench being aligned with the electric field.
 4. The method of claim 1, wherein depositing a plurality of carbon nanotubes in the trenches comprises: spraying a solvent onto a plate under application of an electric field to form a fiber containing nanotubes aligned in a direction of a fiber axis; and inserting the fiber into the trench.
 5. The method of claim 1, wherein forming first and second contacts on respective first and second ends of the trench comprises: etching first and second openings at respective first and second ends of the trench; and depositing a conductor in the first and second openings.
 6. The method of claim 1, wherein the first and second contacts are formed after depositing the carbon nanotubes.
 7. The method of claim 1, wherein forming first and second contacts on respective first and second ends of the trench comprises: etching first and second openings at respective first and second ends of the trench; and depositing a conductor in the first and second openings.
 8. A method of making a stress sensor comprising: forming a trench having a length in a semiconductor substrate of a die; depositing a plurality of carbon nanotubes in the trench, the carbon nanotubes aligned in the direction of the length of the trench; and forming first and second contacts on respective first and second ends of the trench.
 9. The method of claim 8, wherein the first and second contacts are formed after depositing the carbon nanotubes.
 10. A method of making a stress sensor comprising: forming a trench having a length in a substrate of a die; depositing a plurality of carbon nanotubes in the trench, the carbon nanotubes aligned in the direction of the length of the trench; and forming first and second contacts on respective first and second ends of the trench wherein the first and second contacts are formed before depositing the carbon nanotubes.
 11. The method of claim 8, wherein the first and second contacts are formed before depositing the carbon nanotubes.
 12. The method of claim 8, wherein forming the trench in the substrate comprises: forming a photoresist mask on the die, the photoresist mask having an opening; and etching the die in combination with the photoresist mask to form the trench.
 13. The method of claim 8, wherein depositing the plurality of carbon nanotubes in the trench comprises: forming a nanotube suspension in a solvent; dispensing the nanotube suspension into the trench using a pipette; evaporating the solvent; and removing non-aligned nanotubes.
 14. The method of claim 8, wherein depositing a plurality of carbon nanotubes in the trench comprises: forming a nanotube suspension in a solvent; applying an electric field to the solvent to align the nanotubes; and dipping the substrate into the solvent, the trench being aligned with the electric field.
 15. The method of claim 8, wherein depositing a plurality of carbon nanotubes in the trenches comprises: spraying a solvent onto a plate under application of an electric field to form a fiber containing nanotubes aligned in a direction of a fiber axis; and inserting the fiber into the trench. 